1. Field of the Invention
This invention generally relates to solid state neutron detectors and, more specifically, to neutron detectors made with Gd-containing oxide and nitride heterojunctions, and corresponding fabrication processes.
2. Related Art
There are several approaches to detecting and monitoring neutrons, including use of neutron scintillation detectors, 3He detectors, solid-state conversion layer detectors, and neutron-absorbing semiconductor solid-state detectors. In every case, neutrons are captured. Generally, the commercially available neutron detectors based on 3He [1] and CdZnTe [2], [3] are not sensitive enough, are too bulky, have considerable power requirements and are too expensive to be widely applicable for fissile material interdiction operations. This is partly because the operational devices tend to require higher voltages even for the detection of thermal neutrons. In addition, efficiency is largely dependent upon the efficient capture of the neutron, which is 3840 and 2454 barns for thermal neutrons in 3He and Cd respectively. The resulting unstable atom decays by emitting more easily detected radiation, such as an alpha particle. Moreover, these devices are bulky and require more power for operation and signal generation, while at the same time have more limited lifetimes. Furthermore, it is clear that supplies of 3He are in increasingly short supply so less expensive alternatives must be sought.
A problem with this thermal neutron detection method is that neutrons emitted by fissile materials are fast, with an average energy of 1.5 MeV, while the capture cross section is greatest for slow (epithermal to thermal) neutrons. For this reason, neutron detectors based on 3He or Cd need a thick moderating layer (at least several centimeters) in order to thermalize the neutrons to ˜30 meV or less which reduces the efficiency owing to inelastic scattering losses in the moderator. Moreover, these devices are bulky and require more power for operation and signal generation, while at the same time have more limited lifetimes. Furthermore, it is clear that supplies of 3He are increasingly short supply so in expensive alternatives must be sought.
Solid-state neutron detectors may potentially increase efficiency without adding to the overall volume (bulk) of the detection system, provided an appropriate capture material can be used. Semiconductor detectors can be relatively inexpensive, robust and reliable, and exhibit a compact volume, and require less power. Thus, a neutron detector based upon a semiconducting medium could be made portable and powered by batteries with an extended operational lifetime.
An important milestone was recently achieved with the development and demonstration of a 10B-based boron-carbide semiconductor neutron detector [4], [5]. By using B as a capture material within the semiconductor, the device lifetime was greatly increased, and the power consumption greatly reduced. However, owing to 10B neutron capture cross-sections, the system still requires a thick moderating layer to achieve practicable performance. Thus an ideal efficiency and compactness was not realized.
The extremely large thermal neutron absorption cross section of gadolinium (Gd) is an attractive property for creating a high efficiency neutron detector. Natural Gd has a thermal neutron capture cross section of 46,000 barns, while 15.65% abundant 157Gd has a cross section of 255,000 barns [6-10]. Additionally, the Gd cross section remains significant out to neutron energies of about 200 meV [8-10].
This cutoff energy is higher than, for example, boron whose cross section drops greatly above 25-30 meV [8-10] and hence allows the use of less moderating material if detection of high energy neutrons is desired [1,1]. With the significant natural abundance of large cross section isotopes of Gd, isotopic enrichment is not necessary even for a thin film device of fairly modest thickness. Indeed for a 15% Gd doped HfO2 layer, the neutron absorption for 100 meV neutrons is comparable to a boron carbide layer for 30 meV neutrons; requiring a layer in the region of 30-40 microns for the same opacity. Enriched 157Gd is commercially available and may be used to quintuple the absorption, if needed. The 157Gd(n,γ)158Gd and 155Gd(n,γ)156Gd reactions involve the emission of energetic gamma particles which do not significantly contribute to local energy transfer, as well as low-energy X-rays, conversion electrons, and Auger electrons. The conversion electrons are emitted at about 220 keV for 155Gd and through a number of decay channels for 157Gd, of which the 79.5 keV and 182 keV ones are dominant[12-15]; in addition, Auger electrons are emitted at 40 keV (K-shell transitions accompanied by a 44 keV X-ray photon) and 5-8 keV (L-shell transitions). The M-shell binding energy is only 1.8 keV, producing a peak close to the direct 79.5 keV channel.
Thermal neutron reactions with Gd differ significantly from 10B or 6Li interactions because it nearly always results in an (n,γ) reaction, as in:157Gd(n,γ)→158Gd+γ+X-rays+IC e−+ACK e−,  (1)
which leads to the emission of low-energy gamma rays and conversion electrons. 10B or 6Li interactions undergo (n,α) reactions with a large Q-value, such as the 10B(n,α)7Li neutron capture reactions:10B+n→7Li(0.84 MeV)+4He(1.47 MeV)+γ(0.48 MeV)94%  (2)10B+n→7Li(1.02 MeV)+4He(1.78 MeV)6%  (3)
and 6Li(n,α)Tneutron capture reaction:6Li+n→3H++4He2++4.5 MeV.  (4)
The relatively low energy of the conversion electrons produced by the 157Gd (20-40 times smaller than the 10B(n,α)7Li reaction) is the main drawback of using Gd as a neutron detector. This does not necessarily reduce detection efficiency, as long as the current pulses from 79.5 keV and other conversion electrons can be reliably identified. But the devices will require large depletion or charge collection regions of 50-60 microns in the total neutron if the generated signal is to be obtained. We show below that all these electrons can be detected, because Gd-based devices can have much smaller leakage currents (and hence noise) compared to boron-carbide devices. Furthermore the host semiconductors can be fabricated so that while only a few microns is required for neutron opacity, the semiconductor device can retain a fairly large (50 micron) charge collection region. Indeed the host semiconductors are well known to be compatible with having the necessary 1-10 fF charge to voltage amplifier constructed right on the detector heterostructure itself, thus leading to other improvements in noise rejection.
Neutron and other similar heavy particle detectors present an increasingly important component of national safety and security. Ideally, handheld solid-state detectors will allow inspectors to track the shipment of radioactive materials intra-state and inter-state. There is ideally a method by which every ship entering every harbor in the United States, and every vehicle crossing every national boundary, as well as truck weigh stations distributed throughout the National highway system, can be monitored, so that the safety of known shipments of radioactive materials can be documented, and the introduction of unwanted materials can be kept from elements adverse to the interests of a nation. A solid state detector, which permitted both qualitative (i.e., there is radioactive material present) and quantitative (how much material is present) outputs would advance these interests significantly. Representative neutron detectors are disclosed in U.S. Pat. Nos. 6,771,730 and 7,368,794, both of which are incorporated herein-by-reference.
Incorporating Gd into a semiconductor diode detector has many of the potential advantages associated with diode detectors such as an increased number of carriers, compact size, relatively fast timing response and a controllable depletion depth [16]. Hafnium oxide (HfO2) is an obvious choice to incorporate Gd into a diode detector because Gd is readily incorporated into the HfO2 lattice [17-21], with the Gd occupying the Hf sites. The high resistivity and high thermodynamic stability of HfO2 in contact with Si also indicates [22] that a Gd doped HfO2/Si heterojunction has the potential to make a useful device [19-20]. Other possibilities including Gd doped GaN or Gd containing oxide heterojunctions with lithium borates or with B doped Si as well as heterojunctions of Gd containing nitrides to boron nitride will be discussed later. It should be mentioned here that these semiconductor heterojunction diode detectors are fundamentally different from the conversion layer devices based on Gd foils and films [23].
In addition to neutron detection for homeland security purposes, the advantages of the solid state neutron detector, including vibration resistance, high temperature operation and low power consumption, make them desirable for oil well logging. In such a process, a neutron source and a detector are lowered into the well as it is being drilled. The detector measures the amount of neutrons that scatters back from the well's surroundings, which indicates the porosity of the rocks and abundance and quality of the hydrocarbon zones in the geological formations below the Earth's crust.